If you're a committed peace activist, you'll inevitably conclude that there's no hope of stopping aggression / imperialism without an alliance with right-leaning non-interventionists (or "isolationists" if you like the establishment's snarl words).
When you discover this and pursue such an alliance, some of your fellow leftists (inevitably ones who tend to be silent on foreign policy) will balk and lose their shit on account of your right-leaning friends' views on immigration, firearms, planned parenthood, or whatever else.
The Deep State, military industrial complex, and left-oligarchs like Schmidt / Soros / Saban will pour as much money and energy into that wedge as possible: undermining any possible cohesive anti-war movement;
Lather. Rinse. Repeat.